1. Field of the Invention
The present invention relates to a gas sensor that includes a gas sensor element for detecting the concentration of a specific gas component in a gas to be measured. More specifically, the present invention relates to a self-generating type gas sensor in which a standard electrode functions as an oxygen standard with the accumulation of oxygen.
2. Description of the Related Art
A gas sensor of the related art includes a gas sensor element in which different magnitudes of electromotive force are generated depending on the concentration of a specific gas (for example, NOx or oxygen) in the exhaust gas of an automobile or the like. An example thereof is an oxygen sensor including an oxygen pump cell and an oxygen concentration detection cell in which a pair of electrodes sandwiches a solid electrolyte body formed of zirconia. Among them, in the oxygen pump cell, the magnitude or the direction of the current flowing between a pair of electrodes is controlled so that the electromotive force generated in the oxygen concentration detection cell becomes a standard voltage, whereby oxygen is pumped into or out of a gas detection chamber. In addition, based on the current flowing in the oxygen pump cell, the oxygen concentration of the exhaust gas and the air-fuel ratio of the exhaust gas can be detected (e.g., see Patent Document 1).
[Patent Document 1] JP-A-2007-33114
3. Problems to be Solved by the Invention
However, in the above-mentioned self-generating type gas sensor, by causing a minute current to flow in the oxygen concentration detection sensor, oxygen ions move from the detection electrode to the standard electrode, thereby accumulating oxygen in the standard electrode. In addition, a predetermined level of standard oxygen is generated in the oxygen sensor by oxygen accumulated in the standard electrode. Therefore, the standard electrode has been formed of a porous metal in which a mixed paste continuing platinum and zirconia is sintered so as to exhibit both conductivity (for providing an electrode function) and porosity (for accumulating oxygen). Furthermore, such a gas sensor includes a standard lead for electrically connecting the standard electrode with the outside and drawing oxygen, which accumulates in the standard electrode to a predetermined level or greater, to the outside. That is, since balancing the electric connection property and oxygen permeability is required even in the standard lead, the standard lead is integrally formed with a porous metal in the same manner as the standard electrode in the related art.
However, in the standard lead, as the content of the precious metal is increased so as to improve the electric connection property, the porosity and therefore the oxygen permeability decreases. On the other hand, as the content of the precious metal is decreased so as to improve the oxygen permeability and therefore increase the porosity, the electric connection property decreases. Therefore, in the gas sensor of the related art, the balance between the electric connection property and the oxygen permeability in the standard lead must be adjusted with a high degree of accuracy, whereby design of a standard lead is difficult.